A known method of disposing of the regenerative energy from a load in the above type of ac/dc converting circuit will be described referring to FIG. 13, and a known method of initially charging a dc side condenser will be described referring to FIG. 14. The basic operation of a full wave rectifying circuit in the ac/dc converting circuit is disclosed in detail in Japanese Patent No. 7-79548 titled "AC/DC CONVERTING CIRCUIT", for example, and therefore will not be explained herein.
Initially, the method of disposing of the regenerative energy from the load will be described. A main circuit arrangement shown in FIG. 13 includes an ac power supply 1 for the system, ac reactor 2, switching portion 3 in which two reverse parallel circuits of a self-commutated semiconductor switching element, such as IGBT, and a diode are coupled in series, and a rectifying portion 4 that is a series circuit of two diodes. The circuit arrangement further includes a dc side condenser 5, load 6, resistor 7, direct voltage detector 8 for detecting the terminal voltage of the condenser 5, input current detector 9, input voltage detector 10, converter control device 11, comparator 13, level setting device 14, monostable multivibrator 15, and a semiconductor switching element 19 connected in series to the resistor 7. In this arrangement, the switching portion 3 and rectifying portion 4 constitute a full wave rectifying circuit.
In the above-described main circuit arrangement, the electric energy that returns from the load 6 cannot be regenerated into the ac input side, but is stored as a charge in the condenser 5. As a result, the terminal voltage of the condenser 5 increases, and the resultant voltage is applied to both ends of each of the switching portion 3 and rectifying portion 4. It is therefore necessary to limit this voltage to be equal to or lower than allowable values of the elements in these portions 3, 4.
To this end, the comparator 13 compares the terminal voltage of the condenser 5 detected by the direct voltage detector 8 with a predetermined value set by the level setting device 14, and, if the terminal voltage exceeds the predetermined value, the monostable multivibrator 15 operates to produce output pulses for activating the switching element 19. As a result, the discharge current of the condenser 5 flows through the resistor 7, so that the regenerative energy is consumed by the resistor 7, thereby reducing the terminal voltage of the condenser 5.
The method of initially charging the dc side condenser 5 will be now explained. As well known in the art, the initial charging of the condenser 5 is needed to restrict the rush current flowing into the condenser 5 when the ac power supply 1 is turned on.
In FIG. 14, the same reference numerals as used in FIG. 13 are used to identify the same constituent elements as used in the arrangement of FIG. 13. In the circuit arrangement of FIG. 14, a thyristor 22 as a semiconductor switching element is connected between the cathode of the diode in the upper arm of the rectifying portion 4, and the positive electrode of the condenser 5, and the resistor 7 is connected between the opposite ends of the thyristor 22. This thyristor 22 is adapted to be activated in response to an output signal from the comparator 13.
In the circuit of FIG. 14, the thyristor 22 is in the OFF state when the ac power supply 1 is turned on, and the rush current flowing into the condenser 5 is limited by the resistor 7. Once the voltage of the condenser 5 reaches a given value after turning-on of the power supply 1, the thyristor 22 is activated in response to the output signal from the comparator 13, so that the main circuit of FIG. 14 starts operating as a circuit arrangement in which the resistor 7 is substantially eliminated.
In the arrangements of FIG. 13 and FIG. 14, accessory components, such as a semiconductor switching element and a resistor, need to be individually connected for different purposes to the main circuit including the fill wave rectifying circuit, which may undesirably increase the number of components, volume of the apparatus, and the manufacturing cost.